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"The Crown will have to prove beyond reasonable doubt that Vincent Skeen ... swinging out with a broken bottle, turned his mind to the fact that what he was about to do might well kill and he decided to take the risk regardless. The defence says that's simply not the case," she said.

"Vincent would have been hard-pressed to hit Luke in the neck on purpose. He simply swung at a boy who was older, bigger and stronger."

Teenagers had spilled on to the street after a party at a Grey Lynn apartment on the night in question, Crown prosecutor Brian Dickey said.

Neither Skeen nor Tipene had attended the party but they came together when a fight between their respective friends erupted on Great North Rd.

"Mr Skeen stepped into what was supposed to be a one-on-one fight with Luke's friend and another young man," Dickey said.

He told the jury the reason for the initial fight was unclear but appeared likely to be about a girl.

"And at that point matters escalated as Luke, who had been standing by, stepped into the fight and hit Mr Skeen to the side of his head to assist his friend who at this stage had become outnumbered," the prosecutor said.

Tipene knocked the defendant to the ground.

Dickey said Skeen became "enraged" and sought retribution, allegedly finding a beer bottle in the back of a ute and smashing it to form a "protruding, jagged blade".

The Crown said witnesses would give evidence of the defendant inflicting multiple blows to Tipene as though "hammering a nail" or throwing a roundhouse punch.

"One of those blows, perhaps though not necessarily the final blow, was in a slight downward motion to the left side of Luke's throat," Dickey said.

The result was a 12cm wound, which stopped just short of Tipene's spine and pierced his jugular vein.

"Luke was rushed to Auckland Hospital where surgeons tried valiantly to repair damage to his neck ... but by 4.45am he was dead, at 17-years-old, killed by the wound to his neck inflicted by the defendant," Dickey said.

"The question for you in this trial is likely to be: what was the intention or what was the defendant thinking?" Dickey told the jury.

Smith accepted her client did something "very foolish" but could not have appreciated the consequences of his actions.

"If the blow had hit Luke in the chest or his shoulder or arm or just about any other part of his body other than his neck, we wouldn't be here," she said.